This disclosure generally relates to systems and methods for measuring a level of liquid in a reservoir, such as a storage tank or other container. More particularly, this disclosure relates to systems and methods for liquid level measurement using an optical sensor.
The level of a liquid is continuously measured in many commercial and military applications. For example, liquid-level sensors are commonly used in the fuel tanks of aircraft, automobiles, and trucks. Liquid-level sensors are also used to monitor liquid levels within storage tanks used for fuel dispensing, wastewater treatment, chemical storage, food processing, etc.
Many transducers for measuring liquid level employ electricity. The electrical output of such transducers changes in response to a change in the liquid level being measured, and is typically in the form of a change in resistance, capacitance, current flow, magnetic field, frequency, and so on. These types of transducers may include variable capacitors or resistors, optical components, Hall Effect sensors, strain gauges, ultrasonic devices, and so on.
Currently most fuel sensors on aircraft use electricity. For example, existing electrical capacitance sensors require electrical wiring inside the tank, which in turn requires complex installations and protection measures to preclude a safety issue under certain electrical fault conditions. This electrical wiring requires careful shielding, bonding, and grounding to minimize stray capacitance and further requires periodic maintenance to ensure electrical contact integrity.
A simplex (non-differential) optical impedance fuel level sensor based on optical intensity measurement has been proposed which would eliminate all electrical elements. One such optical impedance fuel level sensor comprises two optical fibers spaced apart inside a meniscus tube: a side-emitting optical fiber that transmits light along its length and a side-receiving optical fiber that receives emitted light along its length. The meniscus tube minimizes the sloshing of fuel level. The variable fuel level in the tank produces changes in the optical impedance between the two optical fibers, resulting in changes in the total light received by an optical detector.
However, the aforementioned simplex optical impedance fuel level sensor is susceptible to inaccuracy due to intensity variations along the optical path that are not related to fuel level. These intensity variations may be attributable to one or more of the following factors: (1) temperature variation; (2) surface tension wetting (non-shedding of liquid); (3) fuel gunk buildup on the optical window surface of the fiber sensor elements; (4) ice slush in the lower portion of the fuel tank due to water condensation and cold temperature; (5) fuel surface tilt in a dynamic flight environment; (6) fiber attenuation due to aging; (7) fiber attenuation due to bending; (8) connector attenuation due to alignment; (9) non-uniformity of light emitting along the length of the side-emitting optical fiber due to manufacturing imperfection; and (10) non-uniformity of light received along the length of the side-receiving optical fiber due to manufacturing imperfection.
It would be desirable to provide a liquid level sensor that measures the optical impedance of light propagating through the liquid in a manner that is not corrupted by one or more of the aforementioned sources of intensity variation.